The present invention relates to an improved method of preparing catalysts for use in the oxidation of hydrocarbon to prepare dicarboxylic acids and anhydrides. More particularly, this is an improved method of preparing phosphorus-vanadium catalysts.
Basically, all of the methods disclosed in the prior art seek to obtain the vanadium in a valence state of less than 5. One method of achieving this result is to begin with vanadium in less than the +5 valence state. Another method is to start with vanadium in the +5 valence state and reduce the valency to less than +5. This invention in particular relates to the latter method.
In the reduction method, acids such as hydrochloric, hydroiodic, hydrobromic, acetic, oxalic, malic, citric, formic and mixtures thereof such as a mixture of hydrochloric and oxalic may be used. Sulphur dioxide may be used. Less desirably, sulfuric and hydrofluoric acids may be employed. Other reducing agents which may be employed are organic aldehydes such as formaldehyde and acetaldehyde; alcohols such as pentaerythritol, diacetone alcohol and diethanol amine. Additional reducing agents are such as hydroxyl amines, hydrazine, and nitric oxide.
The reducing agent can also comprise an organic acid or aldehyde reducing agent, preferably, having 1 to 18 carbon atoms, and more preferably, 1 to 8 carbon atoms; such as for example, oxalic acid, citric acid, formic acid, ascorbic acid, malic, formaldehyde, acetaldehyde or mixtures thereof and a coreducing agent comprising a secondary alcohol having 3 to 12 carbon atoms such as, isopropanol, 2-butanol, 2-pentanol, 3-pentanol, 2-octanol, 6-dodecanol, in an aqueous solution of phosphoric acid.
Generally, the reducing agents form oxysalts of vanadium. For example, if V.sub.2 O.sub.5 is dissolved in hydrochloric or oxalic acid, the corresponding vanadium oxysalts are produced. These vanadium oxysalts should have as the salt forming anion an anion which is more volatile than the phosphate anion.
Trivalent phosphorus compounds may also be employed as reducing agents as particularly disclosed in U.S. Pat. No. 3,907,707.
In order to reduce the pentavalent vanadium, the prior art has quite logically employed at least a stoichiometric amount of the reducing agent and where this aspect of the catalyst preparative process has been discussed, an excess of reducing agent (theoretical amount) is preferred, e.g., U.S. Pat. No. 3,907,707 granted Sept. 23, 1975, to Raffelson et al and U.S. Ser. No. 548,163 filed Feb. 7, 1975, by Kerr now U.S. Pat. No. 4,016,105.
The present invention has as an advantage the preparation of catalysts of superior activity. It is a particular feature of the present invention that the catalysts prepared in accordance therewith are improved in conversion and/or selectivity for the partial oxidation of n-butane to produce maleic anhydride. Another feature is the attainment of a more efficient reactor utilization by higher dicarboxylic acid anhydride production per unit of catalyst per hour. A further feature of the invention is the reactor and the hot spot temperatures which are maintained at the same or reduced level as compared to less efficient catalysts prepared with excess of reducing agent.